Ground shaking during Puget Sound earthquakes is characterized by irregularly shaped areas of high intensity ground motion, not all of which are correlated with soil type. Simulation of the 1965 Seattle earthquake using synthetic accelerograms indicates that this irregular appearance is due to focusing of seismic energy at the boundary between bedrock and the overlying glacial sediments and also to the effects of poor quality soils at the surface. Soil effects have been incorporated into a seismic risk map for the Puget Sound area.